doc/config.md: Significant update, less limitations for /etc/finit.d
doc/config.md: Significant update, less limitations for /etc/finit.d

- Detail limitations in separate section
- Focus on syntax early in document
- Overhaul and simplify introduction

Signed-off-by: Joachim Nilsson <583c295fd7602c168ad814279bbc3894ba65f5d6@gmail.com>

troglobit/finit,troglobit/finit